Issue 43

M.P. Tretyakov et alii, Frattura ed Integrità Strutturale, 43 (2018) 146-154; DOI: 10.3221/IGF-ESIS.43.11 151 with the increase in the previously achieved degree of postcritical deformation, which can be considered as an increase of the strength of steel in the necking zone with an increase by the previous deformation. a b Figure 6 : The ultimate stress obtained in tension tests of specimens turned along the entire length of test part ( a ) and specimens turned in the necking area (b) depending to level of the postcritical deformation achieved at beginning of unloading In the Fig. 7 the extension curves of the initial specimens up to failure (solid line) and unloaded after reaching various levels of postcritical deformation (dotted lines) are shown. Strain curve of steel reflecting the change in the true stresses versus engineering strain (dashed line) obtained by the ratio of the load on the specimen to the current cross-sectional area of the specimen (by the minimum diameter), also is presented in the Fig. 7. The changing diameter of the test part Δd of the sample during the stretching was carried out using a three-dimension video system, by measurement of displacements along coordinate z , which coincide with radial direction of specimen. From the given data, it can be seen that the strain curve of true stresses has an increasing type at the postcritical deformation stage, which correlates with the calculated values of the critical stresses obtained by stretching the specimens done according to the scheme 2. Fig. 8 shows the experimental data reflecting the distribution of longitudinal strains ε yy , constructed at regular time intervals, along the length both initial samples ( a and b ), and for cases of specimens grooving along the entire length ( c ) and in the necking area ( d ). Figure 7 : Strain curves of initial specimens at failure (solid line), after unloading at different levels of the postcritical deformation (dotted lines), strain curve obtained with account of the current cross-section area (dashed line); changing of ultimate stress obtained in tests by scheme 1 and 2 and correspondence trend lines (dash-dotted lines).